The Base Flow Index (BFI) is estimated using the BFIHOST model. This uses 29 Hydrology Of Soil Types (HOST) class fractional extents within the catchment. There are separate model coefficients for Scotland compared with the rest of the UK. The form of the equation is:

where $HOST_i$ is the host descriptor value $i$ for the catchment, and $a_i$ is the weight assigned to host class $i$.